Problem: Ben Frank is an electrician who installs lighting fixtures. He charges (in dollars) his clients $15h+40f$ where $h$ is the number of hours worked and $f$ is the number of fixtures installed. How much does Ben Frank charge when he works $7$ hours and installs $9$ fixtures? $\$$
Working $7$ hours and installing $9$ fixtures tells us that $h=7$ and $f=9$. Let's substitute $h={7}$ and $f={9}$ into the expression and evaluate: $\begin{aligned} &\phantom{=}15h+40f\\\\ &= 15({7})+40({9})\\\\ &= 105+360\\\\ &= {465} \end{aligned}$ Ben Frank charges $\${465}$ when he works $7$ hours and installs $9$ fixtures.